1. Field of the Invention
This invention relates to a motor drive device for long-footage film, and more particularly, to such a device for use in cameras wherein a first motor is provided for effecting shutter release, constant-amount feeding of film and shutter charge, and a second motor is provided for effecting take-up of the amount of film subjected to the constant-amount feeding.
2. Description of the Prior Art
Such a motor drive device for long-footage film is capable of effecting single-frame photography and continuous photography, and requires the following sequence of operations to occur in order to effect continuous photography.
First, depression of the operating button for the motor drive device causes the shutter to be released to effect exposure of the film by a mechanism within the camera. After the lapse of the time required for the shutter operation, such as exposure and the like, and the pause time for determining the interval of photography, the first motor M1 is driven to start the next constant-amount feeding of the film while, at the same time, the take-up of the film onto the take-up spool is started by the second motor M2. As soon as the constant-amount feeding by the first motor M1 is completed, there occurs a shift to the operation for effecting the next shutter release. Even if the interval of photography is slowed down, this relationship is invariable with the only exception that the pause time increases.
The motor drive device for long-footage film having such a sequence of operations is provided with a time constant circuit for preventing any excess amount of film from being pulled toward the take-up spool, or for preventing the film from being acted upon by an excess tension at a point of time whereat the take-up has been completed between the constant amount of the film fed by the first motor M1 and the amount of the film to be taken up by the second motor M2, and the time during which the second motor M2 is powered is electrically controlled so that the power supply to the motor M2 is discontinued before the constant-amount feeding by the motor M1 is completed.
However, as the volume of the film is increased to increase the number of frames available for photography, the inertia of the system including the film and the second motor M2 is so greatly increased as to prevent the second motor M2 from being stopped immediately after this motor M2 receives the stop signal from the above-mentioned time constant circuit and thus, a longer time is required before the take-up spool is actually stopped.
Now, the time T from the start of take-up of the second motor M2 (that is, the start of power supply to the second motor M2) until the stoppage thereof differs depending on the diameter of the film roll, and further varies with the amount of film slack at the start of take-up and the take-up velocity (the angular velocity of the take-up spool). Such variation will be discussed in connection with the graph FIG. 1 of the accompanying drawings, wherein the ordinate represents the amount of take-up and the abscissa represents the feeding velocity of the sprocket (indicatd by straight line L.sub.1) which is constant. The amount of take-up each time from the start of take-up until the stoppage of the motor M2 may generally be considered in the form of three models. Solid line L.sub.2 represents the photography occurring when the diameter of the film roll is small, namely, when the number of exposed frames is small. In this case, as seen, the take-up velocity is great because of the small inertia of the film and reaches the film feeding velocity L.sub.2 in a short time, so that the film is taken up under its tight condition with a resultant tendency that an excess amount of film is pulled on toward the take-up spool, but the film is caused to slip by a well-known slip mechanism. After lapse of the time during which the motor M.sub.2 is powered, the film continues to slip for a certain short length of time and then gradually comes to a standstill. A time lag from cut off of power to the second motor M2 to the actual stoppage thereof is short, but the tension of film during the the take-up operation is strong. The time lag in this case is defined as T.sub.2. Broken line L.sub.3 represents the photography occurring in about the intermediate portion of the long-footage film and in this case, the time lag of the motor M.sub.2 is greater in accordance with the increased diameter of the film roll, than in the case indicated by solid line L.sub.2, and the tension of the take-up is smaller than that in the case of the solid line L.sub.2. The time lag in this case is defined as T.sub.3. Dots-and dash line L.sub.4 represents the photography occurring when the unexposed portion of the film is about to terminate. The time lag in this case is also longer than in the case of the Curve L.sub.3, but the tension of the film take-up is smaller than in the case of the Curve L.sub.3. The time lag of this case is defined as T.sub.4. Where motor drive photography is effected with use of conventional motor drive device in which the power supply to the film take-up motor M2 is stopped before the completion of the film supply by the film feeding motor M1 and which is charged with a long-footage film having 100 feet or more length, the take-up spool is apt to be still rotated at the time when the shutter release operation is initiated and the sprocket becomes freely rotatable. This causes the disadvantageous feed or advance of the film during photography, thus inducing vibration in the picture plane.
The inertia of the take-up spool after stopping the power supply to the take-up motor (in other words, the time from the stoppage of the power supply to the take-up motor to the standstill thereof) gets larger in accordance that the diameter of the film wound up on the take-up spool becomes larger, while the tension of the film by the take-up spool becomes smaller in accordance therewith. The resistance (or load) against the tension at the film supply side gets larger in accordance with that the film diameter on the take-up spool gets larger.
The above-mentioned advance of the film during photography (or exposure) is caused in a case where the film take-up force of the take-up spool is larger than the resistant force thereagainst at the film supply side. In such a case, it is necessary to release the shutter after the rotation of the take-up spool, that is, after the rotation of the take-up motor has completely stopped, so that the film feeding at the time when the shutter is released may be prevented.
Where the film take-up force at the film take-up side is equal to or less than the resistant force at the film supply side, the disadvantageous feed of the film described above is not caused even if the take-up motor is rotating. (The rotational energy of the motor in this case is used to tighten the winding of the film on the take-up spool.)